Device for differential image determination

ABSTRACT

A real-time image processing device having only one image memory. Consecutive images are sequentially fed in digital form to a recursive filter. The memory is employed in the recursive filter as a &#34;delay element&#34;, as result of which noise integration and differential image determination between consecutive images take place.

The invention relates to a device for processing images with an imageforming set-up for producing images which are split up into elements, amemory for the storage of image information by elements, a subtractioncircuit for the subtraction of image information from each image elementof previously determined image information in the corresponding imageelement and a playback device for reproducing the information determinedvia the subtraction circuit from each image element.

Such a medical examination device has already been proposed in a paperby R. A. Kruger et al. published in the bimonthly journal "OpticalEngineering", Vol. 17, No. 6, November/December 1978, pp. 652-657. Inthe examination device a fluoroscopic image is converted via an imageintensifier and image pick-up chain into a video-signal which is thendigitized. The digitized image is then stored in one of threevideo-memories. Each video-memory must have the capacity to hold summedimage information emanating from a number of fluoroscopic images. Thefunction of the three video-memories changes cyclically. A weightedimage is determined from two memories which reproduces, with emphasis,the differences between the consecutive images stored in the twomemories. An image information processing method such as this is termedtime-interval differential imaging by the authors of the paper. Thethird memory is regenerated while the differences in the two memoriesare being determined. For this reason the device described in the papermust contain three video-memories. This makes it expensive.

The aim of the invention is to provide an examination device which isconsiderably cheaper but capable of processing the same imageinformation.

The examination device according to the invention is characterized inthat at least the subtraction circuit, the memory, a multiplicationcircuit and an addition circuit form a recursive filter. The additioncircuit has a first input for receiving elements of image informationfrom an image producer and a second input for receiving previouslydetermined image information which is stored in the memory. To this end,an output of the memory is connected to the second input of the additioncircuit in such a way that a signal is fed to an input of the memory,which signal is the sum of the information originating from the imagemultiplied by a factor (α) and the information originating from thememory multiplied by (1-α) (in which 0≦α≦1). The subtraction circuit isconnected on the one hand to the output of the memory and, on the otherhand, to the input of the recursive filter. Only one memory space isrequired for a video-image the invention uses the video-memory as partof a recursive filter. This represents a saving and is therefore ofadvantage.

A preferred design of an examination device in accordance with theinvention is characterized by the fact that the recursive filtercontains one multiplication circuit having an input is connected to aninput of the subtraction circuit which in turn has an input coupled toan input of the addition circuit. An output of the addition circuit isconnected then to an input of the memory and a further input of theaddition circuit is connected to the output of the memory. Thispreferred design has the advantage that the processing of thevideo-information can be matched to different examination situationswith only one parameter (the multiplication factor of the onlymultiplier); it is therefore very flexible.

It should be noted that, in the publication mentioned, an additioncircuit is also provided for each video-memory, to which the output ofthe video-memory is fed back. The purpose of the feedback is, however,to sum image information for each image element from differentsequential video-images with the object of improving the signal-to-noiseratio. In the present invention the addition circuit in the examinationdevice forms part of the recursive filter and thus has a differentfunction.

The invention will be explained on the basis of an example given indiagrammatic form in which:

FIG. 1 shows in a block diagram how image information is processed inaccordance with the prior art,

FIG. 2 shows an examination device designed in accordance with theinvention and

FIG. 3 shows a preferred design of the image information processingsection of an examination device designed in accordance with theinvention.

FIG. 1 shows a block diagram of an image processing device designedaccording to the prior art; 5se is made of three parallel-connectedmemory chains. Each chain comprises a primary adder A₁, A₂, A₃, a memoryMM₁, MM₂ and MM₃, a constant adder, A₁₁, A₁₂, and A₁₃ ; and a multiplierM₁, M₂ and M₃. The outputs of the three multipliers are connected to asumming device A₄. In the three memories, MM₂ and MM₁ to MM₃,sequentially digitized X-ray images are stored. Thus it is possible tosum a number of directly consecutive X-ray images in each memory. Forthis purpose, one output from each memory is connected in feedback tothe primary adder. The purpose of the feedback is to improve thesignal-to-noise ratio of the stored image information. The particularmemory chain to which the digitized information I_(in) is fed depends onsignal inputs E₁, E₂ and E₃ which act to block inputs of the primaryadders.

The output of each memory (e.g. a random access memory (RAM)) isconnected to a constant adder A₁₁, A₁₂ or A₁₃ through which an arbitraryconstant may be combined with the information obtained from that memory.The sum of the memory content I_(i) and the added constant E_(i) is thenfed to the multiplier in which a product is formed with a freely chosenconstant factor k_(i). The products formed in the multipliers are fed tothe summing device A₄. The output I_(m) of the summing device A₄ thenfeeds the value ##EQU1##

With the above-described procedure it is possible, for example, togenerate time-dependent differential X-ray images with, e.g. thedifference being reproduced between the X-ray images (k₁ =-1; k₂ =+1; k₃=0; c₁ =c₂ =0) stored in the memories MM₁ and MM₂ while a third X-rayimage is read into memory MM₃. A disadvantage of the set-up describedfor this purpose is that it requires multiple memory spaces.

The examination device of FIG. 2 has the advantage that only one memoryspace MM₂₀ is needed (the memory capacity of MM₂₀ is equal to that ofthe separate memories MM₁, MM₂ or MM₃). The examination device shown inFIG. 2 contains high voltage source G for supplying the X-ray tube B. Anobject O is irradiated with the radiation X generated by the X-ray tubeB, and a shadow image of object O is formed on the input screen of theimage intensifier II. The shadow image, intensified and reduced in size,is converted into an analog video-signal via a camera tube PU connectedto the output screen of the image intensifier II. An amplifier OA with asampling circuit intensifies and samples this video-signal; subsequentlythe sampled signal is converted into digital form via an analog-digitalconverter ADC₂.

The digitized signal is fed to an image information processor comprisingthe following component parts: multipliers M₂₀ and M₂₁, an adder A₂₀, amemory MM₂₀ and a subtraction circuit V₂₀. Furthermore, the examinationdevice shown in FIG. 2 contains a digital-analog converter DAC₂ and adisplay device (e.g. a TV monitor) MON. The examination device can, ofcourse, also contain a magnetic tape recorder, a recorder for video ordigital signals or a copier/printer for the more permanent registrationof the processed X-ray images.

The image information processing part forms a recursive filter and worksas follows: for each image element a value originating from theanalog-digital converter ADC2 is fed to the multiplier M₂₀ ; there thevalue is multiplied by the value α (0≦α≦1) which has likewise been fedto the multiplier M₂₀. The product is fed to the adder A₂₀ to which thevalue for the same image element already stored in the memory MM₂₀multiplied by a factor (1-α) is also fed. The multiplication isperformed by a multiplier M₂₁ which links the output of the memory MM₂₀with an output of the adder A₂₀. The sum of the two values fed to theadder A₂₀ is stored at the address of the image element. The valueoriginating from the analog-digital converter ADC₂ and also the valuestored in the memory MM₂₀ are fed to the subtraction circuit V₂₀, sothat the difference between the two values is fed to the digital-analogconverter DAC₂ and displayed on the monitor MON.

FIG. 3 shows a preferred design of an information processing component;for the sake of clarity an input of the analog-digital converter ADC₂and an output of the digital-analog converter DAC₂ from FIG. 2 areshown. The processing component contains only a subtraction circuit V₃₀,a multiplier M₃₀, an adder A₃₀ and a memory MM₃₀. The subtractioncircuit V₃₀ is interposed between the analog-digital converter ADC₂ andthe digital-analog converter DAC₂. The output of the subtraction circuitV₃₀ is also connected to the multiplier M₃₀ at which a freely selectedfactor (0≦α≦1) is multiplied with an output signal of the multipliercircuit M₃₀. The product of this circuit is fed to the adder A₃₀ as isalso a value requested at the output of the memory MM₃₀. The requestedvalue is also fed to the subtraction circuit V₃₀. A sum generated by theadder A₃₀ is again fed to the memory MM₃₀. The image informationprocessing parts shown in FIG. 2 and FIG. 3 both have the same filterbehaviour.

Information processing according to the preferred design example shownin FIG. 3 is very flexible, as the information processing can be adaptedto different examination situations (e.g. to the flow-rate of contrastmedium) by changing only one parameter (α). By making a correct choiceof (α) the same delay occurs as with image information processingaccording to FIG. 1.

The examples shown in FIGS. 2 and 3 are based on digital technique. Ifanalog memories, such as charge-coupled information carriers, areemployed for the memories MM₂₀, 30, the information processing can beperformed with full analog technique, so that operational amplifiers canbe used for the subtraction circuits V₂₂, V₃₀, the adder A₂₀, A₃₀ andthe multipliers M₂₀, M₂₁ and M₃₀. If charge-coupled information carriersare used as video-memories and similar techniques are employed forpicking up and converting the X-ray image generated on the output screenof the image intensifier into a "video-signal" (instead of a cameratube), it will be found advantageous to synchronize the "reading out" ofthe image pick-up of the image intensifier and the "shifting" of thecharge in the video-memory of the recursive filter.

Besides being employed in the X-ray examination devices shown in FIG. 2,the image information processing unit can also be used in otherexamination devices employing other penetrating radiations such asinfra-red, nuclear and ultrasonic radiation. Furthermore, the imageinformation processing unit can be used in a closed circuit TV systemfor observation or security purposes, since a change in the imageinformation is displayed in emphasized form on the monitor.

What is claimed is:
 1. In a device for processing images which have beenseparated into image elements of the type which comprises a memory forstoring the image elements and subtraction means for subtractingcorresponding image elements in a current image and in a previouslydetermined image; the improvement comprising:multiplying and additioncircuits connected with the memory to form recursive filter means whichfunction to multiply information received from the current image by afactor α, to multiply image information received from an output of thememory by a factor (1-α), where 0≦α≦1, to add the resulting products onan elemental basis, and to supply the resulting sum to an input of thememory.
 2. The improvement of claim 1 wherein the recursive filter meanscomprise:a first multiplier circuit; a second multiplier circuit; anadder; and the memory; the current image information being supplied to afirst input of the first multiplier, the constant α being supplied to asecond input of the first multiplier, an output of the first multiplierbeing connected to a first input of the adder, an output of the adderbeing connected to the input of the memory, a first input of the secondmultiplier being connected to the output of the memory, a second inputof the second multiplier being connected to receive the constant (1-α),an output of the second multiplier being connected to a second input ofthe adder, and wherein one input of the subtraction means is connectedto receive the current image information and a second input of thesubtraction means is connected to the output of the memory.
 3. Theimprovement of claim 1 wherein the recursive filter means comprise:thesubtraction means; a multiplier, and an adder; a first input of thesubtraction means being connected to receive the current imageinformation , a first input of the multiplier being connected to anoutput of the subtraction means, a second input of the multiplier beingconnected to receive the factor α, a first input of the adder beingconnected to the output of the multiplier, an output of the adder beingconnected to an input of the memory, and the output of the memory beingconnected both to a second input of the subtraction means and to asecond input of the adder.
 4. The improvement of claim 2 or 3 whereinthe memory is a digital memory and the adder, subtraction means, andmultipliers comprise digital circuits.
 5. The improvement of claim 2 or3 wherein the adder, subtraction means, and multipliers compriseoperational amplifiers and wherein the memory comprises analogcharge-coupled devices.